Unfortunately, all components used by iRedMail doesn't support this feature.
You can try to search this tool with Google, or write a custom program to achieve it.
Tips if you want to write a custom program:
*) It's used as a Postfix transport program, just like others defined in /etc/postfix/master.cf.
*) You can try to detect attachment with Postfix body_checks(5) or other method, then send this email to your custom program with 'FILTER' action. Reference: http://www.postfix.org/header_checks.5.html
Many programming languages has built-in email parser modules. For example, Python module 'email'.